In conventional ion cyclotron resonance (“ICR”) mass spectrometers, such as those typically used in connection with Fourier Transform Mass Spectrometry (“FTMS”), charged particles are directed into a magnetic field such that various properties of the particles can be measured. In one application of this technology, as described in U.S. Pat. No. 4,959,543, which is incorporated by reference herein in its entirety, charged particles are subjected to a high voltage pulse and caused to be accelerated to larger radii of gyration relative to the particles' natural radii of gyration. Once excited in this fashion, the charged particles move in circular orbits at frequencies given by the cyclotron equation, ω=qB/m (where B is the magnetic field strength and q/m is the charge-to-mass ratio of the particles). The excited cyclotron motions induce transient signals on a pair of parallel electrodes positioned inside the magnet; the transient signals are a measure of the cyclotron frequency of the particles. In fact, the transient signals are actually a composite of the cyclotron frequencies of all of the ions present in the magnet. By implementing certain Fourier transform mathematics (e.g., a Fast Fourier Transform, or “FFT,” algorithm to extract the frequency and amplitude for each frequency component), these transient signals are converted into an m/z (mass/charge) plot that can be displayed as a mass spectrum.
There are a number of commercially available products that implement this technique; by way of example, the QFT-7 Hybrid Mass Spectrometer, the HiResMALDI FT Mass Spectrometer, the HiResESI FT Mass Spectrometer, and Explorer FT Mass Spectrometer (all available from IonSpec Corporation; Lake Forest, Calif.). Other similar devices are available from Applied Biosystems (Foster City, Calif.), Bruker Daltonics (Billerica, Mass.), and Waters Corporation, under the Micromass® MS Technologies trade name (Milford, Mass.).
A significant limitation of this technology, and of the aforementioned products that implement it, is system efficiency. Conventional FTMS mass spectrometers are configured with a large cylindrical magnet. This is required to produce a uniform magnetic field, and to thereby provide an environment in which charged particles can be provoked to move in circular orbits whose frequencies can be readily measured with the aforementioned technique. However, these devices are configured to only introduce charged particles through one axial end of the cylindrical magnet.
Furthermore, the uni-directional flow implemented in these systems, and which is standard in each presently available mass spectrometer, allows for an ion flow to go through the magnet to a detector, but a significant time gap follows during which the information read by the detector is processed by the system. It is only after this time gap that the detector is ready to receive the next ion measurement.
There is thus a need in the art for an improved system for mass spectrometry that obviates at least some of these limitations of currently available technology.